The process relates to a process for the production of 1,2-dichloroethane, hereinafter referred to as EDC. EDC is primarily used as an intermediate product in the production of monomer vinyl chloride, hereinafter referred to as VCM, which, in turn, is used to produce polyvinyl chloride (PVC). When EDC reacts to form VCM, hydrogen chloride (HCl) is obtained. Hence, EDC is preferably produced from ethylene (C2H4) and chlorine (Cl2) in a manner such as to maintain a balance between the hydrogen chloride (HCl) produced and that consumed in the reactions, as shown in the following reaction equations:Cl2+C2H4→C2H4Cl2 (pure EDC)+218 kJ/Mol  (1)C2H4Cl2 (cracked EDC)→C2H3Cl (VCM)+HCl−71 kJ/Mol  (2)C2H4+2HCl+½O2→C2H4Cl2 (raw EDC)+H2O+238 kJ/Mol  (3)
The process for the production of VCM with an adequate HCl balance, hereinafter referred to as “balanced VCM process”, comprises:                a direct chlorination step, in which a portion of the required EDC is produced from ethylene (C2H4) and chlorine (Cl2) and generated as pure EDC; utilising the heat of reaction produced in this direct chlorination step is a central aspect of the invention;        an oxychlorination step, in which the rest of the EDC is produced from ethylene (C2H4), hydrogen chloride (HCl) and oxygen (O2), and generated as raw EDC;        a fractionating EDC purification step, in which the raw EDC, together with the recycle EDC returned from the VCM fractionation step, is freed from the by-products formed in the oxychlorination and EDC pyrolysis steps in order to obtain a feed EDC suitable for use in the EDC pyrolysis step;        an EDC pyrolysis step, in which the pure EDC is combined with the feed EDC and the resulting EDC mixture is thermally cracked; the cracked gas obtained consists of VCM, hydrogen chloride (HCl) and unreacted EDC as well as by-products;        a VCM fractionation step, in which the pure VCM desired as product is separated from the cracked gas, and the other main substances contained therein, namely hydrogen chloride (HCl) and unreacted EDC, are recovered separately as recyclables and returned to the balanced VCM process as reusable feed in the form of recycle HCl or recycle EDC.        
The chlorine (Cl2) required for the direct chlorination step is normally produced in a sodium chloride-(NaCl) electrolysis plant, with caustic soda (NaOH) with a concentration of about 33% yielded as co-product. Due to the high toxicity of the chlorine (Cl2) produced, transportation over long distances is avoided where possible. Hence, the chlorine (Cl2) required for the direct chlorination step is often produced in the vicinity of the direct chlorination unit.
It is commonly known that the minor constituents formed in the cracked gas during the EDC pyrolysis step reduce the purity of the VCM product. Removing these constituents in order to improve the purity of the VCM is costly. Hence, cracked EDC which has been largely freed from such impurities is used in the EDC pyrolysis step. Of the large number of techniques that exist for preventing or, if necessary, removing such detrimental by-products and/or constituents, reference is made in particular to patent specification WO 01/34542 A2, and especially to the state of the art described therein. This shows that the heat released when ethylene (C2H4) and chlorine (Cl2) react to form liquid EDC during the direct chlorination process is sufficient to operate the purification columns for the EDC produced in the “balanced VCM process”.
However, a disadvantage of the process described therein is that the heat of reaction used to heat the purification columns requires the withdrawal of a corresponding quantity of heat in order to condense the vapours. This is done in accordance with a conventional state-of-the art technique, usually by means of cooling water, which must be provided in large quantities. However, even if the purification columns are not operated by the off-heat arising from the heat of reaction from the direct chlorination unit, cooling water must be provided in large quantities in order to condense the vapours from the purification columns.